Chris, It depends on the type of orchid and the type of medium you are using. Epiphytes obtain nutrients from the air so the medium must allow air penetration. Chunky bark allows more air and takes longer to break down. Once the medium looks more like dirt than bark, it's time to repot. As for the type of orchid, Phals don't like change so avoid repotting them unless absolutely necessary. Every two years under normal conditions is about right. Cattleyas are a bit more complicated. Unifoliates (one leaf per pseudobulb) are easy to work with and can be repotted every year. Bifoliates require extra care and do not like to be repotted so be very careful. Bottom line is do some research on your plant and understand its care requirements.
Jim
